Collimated displays systems have long been used in wide-bodied aircraft simulators. The primary components of collimated displays are at least one projector, a projection screen, typically spherical, placed above and in front of the cockpit or cab of the simulator on which the projector projects an image, and a collimating mirror in front of and wrapping around the cockpit or cab for providing a large horizontal field-of-view. The projection screen can be either front or rear projected, and the projected image is viewed via the collimating mirror.
A front projection system has a number of advantages, as compared to a back projection system. Front projection screens are generally more economical to produce, and eliminate the problem of specular cross reflection prevalent in back projection curved screens with a large horizontal field-of-view. Such a cross reflection can degrade image quality, particularly resolution and contrast ratio.
The screen in a front projection system is generally more economical to produce as compared to a back projection screen. This is because in a front projection screen the incident rays reflect directly from the screen surface, thus requiring only one high optical quality surface. In a rear projection screen, the projectors project onto the concave surface of the screen forming an image on the outer convex surface, thus the screen requires two surfaces of high optical quality, and a highly transparent bulk material.
In a rear projection display system, in order to have a clear line of sight to the screen, the projectors must be positioned around the cockpit or cab being simulated. This presents some difficulty in placing a large cockpit, or cab, in a multi-projector system. Unlike in rear projection display systems, the projectors in a front projection display system can be placed outside the screen volume and project an image onto the convex outer surface of the screen. Thus, interference between the cockpit and/or cab is not generally a concern, and a wide range of cockpits and/or cabs can be placed in the simulator without having to place the projectors in a location that is difficult to access for servicing.
Front projection display systems employing spherical screens, however, suffer from diminished brightness and resolution at the top and bottom edges of the screen. This is due to the large incident angle of the impinging rays at these locations.